As concern for the environment has been mounting in recent years, improvement in fuel economy for cars has been in increasing demand. Because the improvement in fuel economy of cars is influenced greatly by characteristics of tires, it has been desired to develop rubber compositions for use in tires satisfying such demand. On the other hand, grip on a wet road surface, that is, “wet grip”, has also been regarded as important from the viewpoint of driving safety. Improvements in fuel economy characteristics require tires to be reduced in rolling resistance. However, reduction in rolling resistance results in lowering of wet grip.
As one of measures to develop tires having low rolling resistance while retaining adequate wet grip, studies have been pursued on coupling agents for rubber/carbon black which can allow not only improvement in dispersing capability of carbon black in a rubber composition as a raw material but also strengthening of chemical bonds between carbon black mixed as a rubber reinforcement and rubber molecules.
Many of what have been developed as coupling agents for rubber/carbon black are amino group-containing compounds. This is because the acid-base interaction between carbonyl and carboxyl groups present on the carbon black surface and the amino groups has been expected to produce a bonding effect.
For example, it has been reported in Patent Document 1 to utilize dialkylamino group-containing sulfur compounds as coupling agents for rubber/carbon black.
In addition, Patent Document 2 has reported the use of organic sulfide compounds having quaternary ammonium salt structure as coupling agents for rubber/carbon black, utilizing an action of an iminium ion group which is one of quaternary ammonium salts and is firmly bound to carbon black surface.
In each of these cases, however, the effect of lowering rolling resistance of tires obtained in vulcanization and molding steps has not yet reached to a level of satisfaction.